This invention relates generally to radiation detectors of the scintillating type, and more particularly to methods and apparatus for coupling a scintillator to a photo sensor.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
At least one known detector in CT imaging systems comprises a plurality of detector modules, each having a scintillator array optically coupled to a semiconductor photodiode array that detects light output by the scintillator array. These known detector module assemblies require an adhesive bonding operation to assemble. The photodiode array and scintillator must be accurately aligned with an alignment system, using a plastic shim to set a gap between the photodiode and scintillator arrays. After alignment, the four corners of the assembly are xe2x80x9ctackedxe2x80x9d together with an adhesive to hold the alignment. The tack is cured, and the thin gap between the photodiode and scintillator arrays is filled by dipping the assembly into an optical epoxy adhesive, which wicks into the entire gap. The epoxy is cured, and the scintillator is thus xe2x80x9cepoxiedxe2x80x9d to the diode array. Thus, in a xe2x80x9cfinishedxe2x80x9d detector module the photodiode array and the scintillator array are separated by a solid, inflexible, non-compliant material. (A detector module having epoxy that is still undergoing curing is not considered a xe2x80x9cfinishedxe2x80x9d detector module.)
This standard scintillator assembly and assembly process suffers from a number of disadvantages. First, the thermal coefficient of expansions of the semiconductor photodiode array and the scintillator array are somewhat different. As the ambient temperature changes during operation of an imaging system, thermal and dimensional stress occurs. Other factors that can result in thermal and dimensional stress are temperature changes during shipping and/or storing, slight air conditioning and humidity changes, and changes in operating conditions. As a result of these stresses, breakage can occur at the interface between the diode and the epoxy, within the epoxy itself, or between the epoxy and the scintillator. Breakage can also occur within the scintillator array or semiconductor diode array itself. Such breakage can be microscopic, in which case, light transmission efficiency is affected, or catastrophic, resulting in destruction of the detector module. It would be desirable to reduce the frequency of breakage, whether microscopic or catastrophic, to increase the reliability of the scintillator assembly and of instruments using scintillator assemblies. In addition, the photosensor array and scintillator array cannot readily be separated. Thus, parts of the detector module cannot be recovered from scrap assemblies. It would therefore be desirable to provide a detector module that is more amenable to scrap recovery procedures.
There is therefore provided, in one embodiment of the present invention, a finished detector module assembly suitable for use in a computed tomography (CT) imaging system. The detector module assembly includes a substrate; a photosensor array mounted on the substrate; an array of scintillators optically coupled to the photosensor array and separated therefrom by a gap filled with either air or a compliant clear film; and a flexible electrical cable electrically coupled to the photosensor array.
Among other advantages, the scintillator array and the photosensor array of the above-described detector module embodiment can be readily separated, thus allowing recovery of parts in scrap assemblies.
In addition, this and other embodiments of the invention provide various combinations of additional advantages, including an improved coupling process, low cost due to a simpler manufacturing process with fewer steps, high light output, lower cross talk, improved gain uniformity, and reliability and lifetime.